1. Field of the Invention
The present invention relates to flexible printed circuit boards (PCBs) and, particularly, to a printed circuit board substrate and a printed circuit board having the same.
2. Description of Related Art
Nowadays, flexible printed circuit boards (FPCBS) are widely used in portable electronic devices such as mobile phones, digital cameras, and personal digital assistants (PDAS). In some electronic devices, certain parts are movable relative to a main body. In these electronic devices, FPCBS can maintain an electrical connection between the main body and the movable parts due to their flexibility.
FPCBS can be manufactured by at least a base film using a wetting process, such as cleaning, developing, etching, electro-plating, etc. The base film includes an insulation film and a copper film. For example, a process for manufacturing electrical traces of the FPCB includes the following steps. Firstly, the base film is cleaned in a cleaning solution to remove surface oils of the copper film. Secondly, a photo-resist layer is formed on the surface of the copper film. Thirdly, the photo-resist layer is exposed to a light beam using a mask having a predetermined pattern. Thus, one portion of photo-resist layer is covered by the mask, and the other portion of photo-resist layer is not covered and irradiated by the light beam. When the photo-resist layer is made of a positive photo-resist, the uncovered photo-resist layer (i.e. the exposed photo-resist layer) is changed to be soluble in a developing agent. Fourthly, the base film having the photo-resist layer is developed in the developing agent. During the developing process, the exposed photo-resist layer is dissolved in the developing agent, and the residual photo-resist layer forms a patterned photo-resist layer. Thus, one portion of the copper film is covered by the pattered photo-resist layer, and the other portion of the copper film is exposed to the outside. Fifthly, the base film having the patterned photo-resist layer is arranged in an etching solution, and the copper film not covered by the photo-resist layer is dissolved by the etching solution. As a result, the residual copper film covered by the photo-resist layer forms a copper trace. Finally, the photo-resist layer covering the copper trace is eliminated, thereby obtaining the desired electrical traces of the FPCB.
In the process mentioned above, the base film is exposed in liquid solutions (e.g., the cleaning solution, the developing agent, the etching solution) repeatedly, and the liquid solution may inevitably penetrate into the base film. Thus, an original characteristic of the base film may be changed, thereby affecting the quality of the FPCB manufactured by such base film. Furthermore, when the copper film is made into copper traces, molecules (e.g., water molecules) or other ions of the liquid solution may migrate from the insulation film to the copper traces to react with the copper traces. That is, an ion migration phenomena has happened between the insulation film and the copper traces. As a result, a quality of the copper traces may be affected, e.g., causing an open circuit phenomena or a short circuit phenomena.
What is need, therefore, is PCB substrate which can resist/avoid the happening of the ion migration phenomena. What is also need is a PCB having the PCB substrate.